1. Field of the Invention
The present invention relates to a catalyst for the synthesis of ammonia from hydrogen and nitrogen.
2. Description of Related Art
The ammonia synthesis catalyst play an important role in the manufacture of ammonia. It effects both the economics and operating conditions of ammonia plants.
Industrial catalysts for ammonia synthesis must satisfy a number of requirements: high catalyst activity at the lowest possible reaction temperatures in order to take advantage of the favorable thermodynamic equilibrium at low temperatures, good resistance to poisoning by oxygen-, chlorine- and sulphur-containing compounds, long life and high mechanical strength.
In order to save energy the synthesis pressure has been steadily reduced from 250-350 bar to 80-150 bar during the last 30 years due to optimized operation of the synthesis loop made possible by new technology, especially for fabrication of the converter. Improvements in the front end has drastically reduced the content of catalyst poisons (oxygen-, sulphur- and chlorine-containing compounds) in the gas inlet to the converter. However, the drop in synthesis pressure necessitates a three- to four-fold increase in the catalyst volume. The demands upon the catalyst quality has also increased. Apparently small improvements in the catalyst activity may lead to major improvements of the operation of a modern ammonia synthesis loop.
The ammonia synthesis catalyst precursor, the unreduced catalyst, is manufactured by melting iron oxides, mainly magnetite, and oxides or carbonates of Al, K, Ca and Mg, the so-called promoters. The melting is performed in electrical furnaces at a temperature of above 1600xc2x0 C. The ratio Fe2+/Fe3+ in the melt is normally in the range 0.5-0.75. The melt is then poured into metal trays where it solidifies and cools. The solid precursor is then broken up, crushed and sieved to obtain the required grain size.
The precursor is reduced to active iron catalyst xe2x80x9cin situxe2x80x9d in the ammonia synthesis converter or used to manufacture the prereduced catalyst by a complete reduction under optimised conditions in a prereduction reactor. This material is pyroforic, but after a skin oxidation of the active surface with oxygen, it can be handled safely in air at ambient temperatures.
The promoters mentioned above, containing Al, K, Ca and Mg, are essential for the formation of a large surface of iron in the reduced catalyst and for the kinetics of ammonia formation.
Since the development of the catalyst started, about 90 years ago, the concentration of these promoters have been optimized to obtain maximum activity and to meet the additional requirement mentioned above. Further improvements seem only possible by adding new promoters, or new combination of promoters.
Cobalt oxide has been used as such a new promoter.
From U.S. Pat. No. 3,839,229 it is known an ammonia synthesis catalyst where cobalt oxide is used as a promoter. The catalyst consists essentially of a solid solution of iron oxide and cobalt oxide and said cobalt oxide is present in an amount of 5 to 10 percent by weight expressed as cobalt. In the catalyst there is further included a promoter selected from the group consisting of alumina, silica, zirconia, magnesia, lime (CaO), potassium oxide and rare earth metal oxides.
Another possible promoter is titanium oxide. Only few catalysts using this promoter have been previously described. S. A. Abdukadyrova et al. have in xe2x80x9cTr.Mosk.Khim.xe2x80x94Tekhnol.Inst. (1970), No. 2, 122-5xe2x80x9d, described that titanium oxide improves the thermal resistance but reduces the activity.
Titanium oxide used as a structural promoter is described by M. E. Dry et al in xe2x80x9cJournal of Catalysisxe2x80x9d, 6, page:194-199, 1966. Titanium oxide is not so effective as aluminium oxide with regard to surface area. This area and the volume of chemisorbed CO normally increase with promoter content, but in contrast to the other promoters (Al2O3, MgO, CO) TiO2 had a maximum in the volume of chemisorbed CO at about 0.5 g atom cation per 100 g atom Fe.
The main object of the present invention was to develop a catalyst for ammonia synthesis with improved activity.
Higher activity could be utilized in two ways: High reaction rate at high ammonia concentrations or higher activity at low temperatures.
The catalyst activity maybe expressed as a rate constant in a rate equation for the synthesis reaction. The rate is a function of temperature, pressure and the composition of the gas, and decreases rapidly with increasing ammonia concentrations. Thus a large fraction of the catalyst volume in a synthesis converter will have ammonia concentration approaching the exit concentration. Hence, to improve the efficiency, increasing the reaction rate at high ammonia concentration is of perticular interest.
As the equilibrium temperature decreases with increasing ammonia concentration, searching higher conversion by increasing the reaction rate at high ammonia concentration also means searching a catalyst will higher activity at low temperature.
The inventors have manufactured a lot of samples of iron oxide based catalysts with various promoters in various concentrations to improve the catalyst activity. In addition to the conventional promoters mentioned above, new promoters have been tested, in particular cobalt- and titanium oxide.
The inventors found that the reaction rate at high ammonia concentration increased with 10-20% when both cobalt- and titanium oxide were used as promoters together with the conventional promoters.
The most preferred catalyst was achieved when the concentration of cobalt was between 0.1% and 3.0% by weight of metal and when the concentration of titanium was between 0.1% and 1.0% by weight of metal.
The atomic ratio Fe2+/Fe3+ was between 0.5-0.65.
The present invention will thus in its widest scope comprise a catalyst for the synthesis of ammonia from hydrogen and nitrogen consisting of iron oxides and promoters where the promoters comprise oxides of both cobalt and titanium in addition to Al, K, Ca and Mg oxides.
The invention will be further explained in the example.